feat: implement full session recording with muxing support

- Add session recording metadata and mux filter builder in src/recorder/sessionRecording.ts.
- Update SegmentMetadata to include recordingSessionId in src/types.ts and src/recorder/metadata.ts.
- Modify recorder lifecycle to track sessions, register segments, and finalize recordings on stop.
- Create tests for session recording functionality in tests/recorder/sessionRecording.test.ts and tests/recorder/metadata.test.ts.
- Document session recording design and implementation plan in docs/superpowers/specs/2026-05-16-session-full-recording-design.md and docs/superpowers/plans/2026-05-16-session-full-recording.md.

tests/recorder/metadata.test.ts

@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import { createSegmentMetadata } from "../../src/recorder/metadata";
+import type { SegmentState, UserMetadata } from "../../src/types";
+
+describe("createSegmentMetadata", () => {
+  const user: UserMetadata = {
+    userId: "user-1",
+    username: "Alice",
+    tag: "Alice#0001",
+    displayName: "Alice",
+    avatarUrl: "https://example.com/avatar.png",
+    bot: false,
+    roles: [],
+    highestRole: null,
+    joinedTimestamp: null,
+  };
+
+  const segment = {
+    index: 0,
+    startTime: 1500,
+    endTime: 2500,
+    filename: "/recordings/user-1/1500.ogg",
+    jsonFilename: "/recordings/user-1/1500.json",
+    oggStream: {} as any,
+    out: {} as any,
+  } as SegmentState;
+
+  it("includes shared recording session id", () => {
+    const metadata = createSegmentMetadata(
+      user,
+      segment,
+      "user-1-1500",
+      "guild-voice-1000",
+      1000,
+      5000,
+    );
+
+    expect(metadata).toMatchObject({
+      sessionId: "user-1-1500",
+      recordingSessionId: "guild-voice-1000",
+      sessionStartTime: 1000,
+      startTime: 1500,
+      endTime: 2500,
+    });
+  });
+});